Cemented carbides for metal cutting have been used for almost 70 years. All the time improvements have been made and higher productivity has been achieved. One of the biggest inventions in this area was the coatings with thin layers of TiC, TiN, Al.sub.2 O.sub.3 etc., which have increased the metal removal rate of such tools considerably. The coatings have also been developed by techniques including initial high temperature chemical vapour deposition (HT-CVD) towards lower deposition temperature (MT-CVD) and also Physical Vapour Deposition (PVD). The thickness and the adherence of the coatings have been improved as well which have changed the compositions for the cemented carbide substrates. Previously these substrates often formed an active part of the cutting tool. However, today the main function of the substrate material is to carry a coating, with the coating being the active cutting material. Once the coating is worn out, the coated substrate, often in the form of a removable insert, is simply discarded.
Substrate developments have included reducing the content of cubic carbides, i.e., towards WC--Co-based cemented carbide substrates. These developments lead to a demand for finer WC grain size in the sintered cemented carbide than previously attained.
Extremely fine-grained WC--Co cemented carbides have been developed for drilling of composite printed circuit boards and similar applications. Here not only submicron but also so called `nano-sized` materials are available. The limit for `nano-sized` is not defined in detail, but up to 200 nm (0.2 .mu.m) is often considered as nano-size. Special production methods are used for these types of materials.
This invention relates to WC--Co-based cemented carbides produced from raw materials made via `traditional` ways, i.e. tungsten carbide powder produced separately by carburizing of tungsten metal powder or tungsten oxide with carbon and cobalt powder. Gas-carburizing is of course included. The precipitation of a cobalt salt on the surface of tungsten carbide followed by reduction to metallic cobalt is consequently excluded.
The sintered mean WC grain sizes for alloys with improved properties if produced via the present invention are in the area 0.6-1.6 .mu.m, preferably 0.6-1.4 .mu.m. Also 0.4 .mu.m WC alloys can advantageously be produced according to the present invention.
For submicron material, grain growth inhibitors must be used: Cr.sub.3 C.sub.2 and/or combinations of VC+Cr.sub.3 C.sub.2 can be used for finer grain sizes.
All cubic carbides in Groups IV and V of the periodic table act as grain growth inhibitors for WC--Co-alloys: TiC, ZrC, HfC, VC, NbC, and TaC. In addition, the hexagonal Mo.sub.2 C and the orthorombic Cr.sub.3 C.sub.2 of Group VI act as grain growth inhibitors. For WC--Co alloys with a sintered mean WC grain size of 1.0-1.6 .mu.m, TaC is a very common grain size stabilizer/grain growth inhibitor, NbC is also often used in combination with TaC. Mo.sub.2 C can be used as well, both in the submicron and micron grain size area (0.8-1.6 .mu.m).
The traditional way to produce cemented carbide is to put the desired proportions of WC, Co and grain growth inhibitors, if any, and a pressing agent like PEG or A-vax, in a wet ball mill with milling bodies of WC--Co (in order to avoid unwanted impurities in the material) and to extensively mill this mixture in alcohol/water or any other milling liquid. The final grain size of the tungsten carbide is determined during this process. The tungsten carbide is often strongly agglomerated, and this is also true for the cobalt powder. The milling process is often very long in order to:
1. Determine the final grain size of the tungsten carbide. PA1 2. Get an even dispersion of the grain growth inhibitors to avoid grain growth in any part. PA1 3. Have the cobalt evenly dispersed to avoid porosity and cobalt lakes in the sintered material. PA1 1) Wear of the milling bodies PA1 2) Wear of the inner walls of the mills (high maintenance cost) PA1 3) Investment costs in a lot of mills to produce the desired amount of material PA1 (a) providing a WC powder with a round morphology; PA1 (b) providing a Co powder alloyed with at least one grain growth inhibitor; and PA1 (c) mixing the WC powder and the Co powder.
This long milling time is detrimental for at least the following reasons:
A long milling time will also create a very wide distribution in grain size of the milled WC particles. The numerous consequences of this broad distribution include: high compaction pressure with high deflection at unloading of the punch and high risk for cracks with modern complicated geometries, and the formation of unfavourable morphologies of the sintered WC grains (triangular, prismatic etc) resulting in low toughness (transverse rupture strength).
After milling, the slurry must be dried, often in a spraydryer, to get a free-flowing powder. This powder is then pressed and sintered to blanks followed by grinding to the final dimensions, and often coated.